Since recent developments in the fields of horizontal drilling and multistage fracturing many Exploration and Production (E&P) operators have experienced difficulties utilizing current technologies to mill or drill the balls and ball seats out of the ball frac sleeves of an open hole ball-type frac liner system completed into a formation or reservoir. The restriction caused by these balls and ball seats prevent optimal productivity of the well and can prevent the E&P companies from entering the liner of the wellbore. Recent developments indicate that a work-over or intervention is required to remove the restrictions (balls and seats), to investigate inflow (production log or production evaluate), to re-stimulate the reservoir, and/or remove blockages such as sand or formation material.
Currently, the technology being used in these situations is typically conventional coiled tubing, water and nitrogen mixtures, and mud motors equipped with drill-bits or mills. These systems can increase the diameter of the liner by removing balls, seats, or other obstructions to achieve a maximum inner diameter of the liner. Current processes, however, create an over-balanced effect/position on the reservoir which in turn can lead to a loss of work-over fluids, such as into the formulation. A loss of work-over fluids can result in the undesired effect of frac proppant (sand) coming out of suspension and “sanding-in” tools and tubing so that they cannot be removed. Sanding-in can result in the entire loss of tools, expensive fishing requirements, and potentially the loss of production from the well which can no longer be accessed. This over-balanced effect can also lead to formation damage resulting in reduced inflow from the formation. The wellbore is often left debris still present and not cleared from the liner, including solids from the seats, frac proppant (sand) and formation fines. This limits the E&P companies from operating the well at its maximum productivity and interferes with the gathering of valuable data that would facilitate optimal development of a given field.
Mixture of water and gas, such as nitrogen, is often circulated downhole to reduce hydrostatic preserve and lift debris to surface.
For E&P companies who are presently doing these operations, the cost and supply of nitrogen can seriously impact the economics and overall outcome. Safety is also major concern for E&P companies using current systems and the operations environment can be categorized as moderate to high risk. One reason for the safety concern is that the injection lines, coiled tubing, and return lines, containing the highly compressible nitrogen can be under extreme pressure. If a pressurized line or tubing is to part or break, the energy stored in the volume of the lines explosively discharges. This sudden release can cause the lines to whip uncontrollably until the energy has bled off. The uncontrolled movement of the lines can, in turn, contact and injure personnel and/or damage other equipment. The choice fluid, a liquid/gas mixture, typically used during current operations is low in density to maintain high velocity. However, in turn, it is also known to wash out the surface iron (coiled tubing reel), flow back vessel manifolds and connections.
Accordingly, there is a need to provide apparatus and methods for clearing a wellbore that can overcome the short-comings of the prior art, such as unstable job economics, potential for formation and equipment damage, and unsafe work environments.